chore: regenerate README (#786)

This PR was generated using Autosynth. 🌈


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

```
2021-01-07 21:33:13,706 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-07 21:33:14,656 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/9a709830-b932-4666-8dc4-bbdfaa5b2500/targets

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

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "9b94c6ef54776fdb8868acf04e371599b7500d57"
+        "sha": "fa353b35d20b6e44e6870ed86f5c55ed843ef32c"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a3e990f3545dc8ccd384a75d20ce9cb185ca6a28"
+        "sha": "41a4e56982620d3edcf110d76f4fcdfdec471ac8"
       }
     }
   ]

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>16.2.0</version>
+      <version>16.2.1</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>