chore: regenerate README (#621)

This PR was generated using Autosynth. 🌈


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

```
2020-11-12 23:31:22,776 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-11-12 23:31:23,743 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/e3433206-81b8-4d59-a10c-147a0dedd73d/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": "8eea5254b0542c7c36c2428bc321fe95a36bdade"
+        "sha": "5be4d9cc62422120e24ecc25fd9f2d14500f0ad5"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "7db8a6c5ffb12a6e4c2f799c18f00f7f3d60e279"
+        "sha": "e89175cf074dccc4babb4eca66ae913696e47a71"
       }
     }
   ]

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>15.0.0</version>
+      <version>15.1.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>