chore: regenerate README (#530)

This PR was generated using Autosynth. 🌈


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

```
2020-10-20 23:01:18,465 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-20 23:01:19,448 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/bd2afeef-c881-4591-b19b-b940b0b5a699/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": "273f40c0b8b86aea1e1c9f54ca13723d4ce97298"
+        "sha": "8d15a5801ab5693d917a6f46d908a5d9681ca02f"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "d5d03413e1879108b5ade8839ce38de01be652da"
+        "sha": "6abb59097be84599a1d6091fe534a49e5c5cf948"
       }
     }
   ]

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>12.1.0</version>
+      <version>13.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>