chore: regenerate README (#1146)

This PR was generated using Autosynth. 🌈


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

```
2021-05-07 16:11:25,389 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-07 16:11:26,780 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/2ec5559e-6096-4620-9d59-9fa7bb76fd20/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "fc3335c55eb504827e835b6e5a1d6fc5dea2c124"
+        "sha": "223baa57ed548d740ee7817593ef52e1c87ff3ae"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "06a8cd0ff7e81b05e6c503eab510ec622384caa7"
+        "sha": "046994f491c02806aea60118e214a9edd67f5ab7"
       }
     }
   ]

README.md

@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-spanner'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner:6.4.0'
+compile 'com.google.cloud:google-cloud-spanner:4.0.3'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.4.0"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "4.0.3"
 ```
 
 ## Authentication