docs: fix guide on enabling source map generation for iOS (facebook#4832)

Author: floydkim

docs/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.77/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.78/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.79/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.80/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.81/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```
 

website/versioned_docs/version-0.82/debugging-release-builds.md

@@ -61,7 +61,7 @@ To enable source map generation:
 - Above the other exports, add a `SOURCEMAP_FILE` entry with the desired output path.
 
 ```diff
-+ SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map";
++ export SOURCEMAP_FILE="$(pwd)/../main.jsbundle.map"
   WITH_ENVIRONMENT="../node_modules/react-native/scripts/xcode/with-environment.sh"
 ```