fix typo

Author: gyuho

README.md

@@ -8,7 +8,10 @@ protodoc generates Protocol Buffer documentation.
 ```
 go get -v -u github.com/coreos/protodoc
 
-prodoc ./parse/testdata --language-options="Go,C++,Java,Python" --title="testdata" --target-path="./sample.md"
+protodoc ./parse/testdata \
+	--language-options="Go,C++,Java,Python" \
+	--title="testdata" \
+	--target-path="./sample.md"
 ```
 
 Note that parser only understands the minimum syntax