chore: regenerate README (#640)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2020-11-17 09:41:32,582 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-11-17 09:41:33,601 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/005ecbdd-e849-49dc-b49e-0b895e2d2872/targets

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Author: yoshi-automation

.github/readme/synth.metadata/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "b5150b57108ffdcb054a7b4f2dfa3daf840dfb29"
+        "sha": "54de51438bac14c66e241fe68a341a47e641c5f2"
       }
     },
     {

README.md

@@ -45,11 +45,11 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:3.0.3'
+compile 'com.google.cloud:google-cloud-spanner:3.0.4'
 ```
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.0.3"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "3.0.4"
 ```
 
 ## Authentication