chore: regenerate README (#755)

This PR was generated using Autosynth. 🌈


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

```
2020-12-16 00:54:12,960 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2020-12-16 00:54:13,952 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/5f46b5bb-b785-4c25-89bf-bc04d8e3e839/targets

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

Author: yoshi-automation

Diff for: .github/readme/synth.metadata/synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "1de6c28ccea569cc8a4e8dc33e318bafb4211539"
+        "sha": "cadbee6b3a0e4979af94d631bd8d1b6b14f013c2"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "996775eca5fd934edac3c2ae34b80ff0395b1717"
+        "sha": "3f67ceece7e797a5736a25488aae35405649b90b"
       }
     }
   ]

Diff for: README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>16.1.0</version>
+      <version>16.2.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>