chore: regenerate README (#912)

This PR was generated using Autosynth. 🌈


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

```
2021-02-24 19:42:39,749 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-02-24 19:42:41,407 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/90ac9e20-0981-41c1-b8d2-28b5f624992b/targets

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

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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "4a3b95501b152a50cb9d938b524ce1f11e476d1b"
+        "sha": "e666ccea29dcefa90246cc73b08dbc2ce1007225"
       }
     },
     {

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>16.4.0</version>
+      <version>17.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -45,7 +45,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:16.4.0')
+implementation platform('com.google.cloud:libraries-bom:17.0.0')
 
 compile 'com.google.cloud:google-cloud-spanner'
 ```