chore: regenerate README (#939)

This PR was generated using Autosynth. 🌈


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

```
2021-03-08 23:42:25,270 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-08 23:42:26,208 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/dd0097d9-634c-4942-b350-6889afa25c3f/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "c2ab0c70a2992a089a1e5d65d850a8f235728972"
+        "sha": "9c3d4f621023546466d79bec1c5da83e2dbdc84a"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "2e7bd5cc66a3f0b90ce1c150eb35c89eeb9872d6"
+        "sha": "52204b292dacd3f907da614db606ed6eac8f865d"
       }
     }
   ]

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>4.0.2</version>
+  <version>5.0.0</version>
 </dependency>
 
 ```