chore: regenerate README (#585)

This PR was generated using Autosynth. 🌈


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

```
2021-08-31 04:52:01,965 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-08-31 04:52:03,388 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/2e7ba583-448b-4508-a2b5-5d04cd343c74/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner-jdbc.git",
-        "sha": "c4621747c1d02e2abc1d5a01c0c0870399bbfca8"
+        "sha": "7b341394dfe0ee7cae5131681ee72d9750eccce1"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "31728d872f50e439ded2f67fa8d67955b26a4e35"
+        "sha": "dfd55ad78a700acf987d592c8279789b1319b8c5"
       }
     }
   ]

README.md

@@ -26,7 +26,7 @@ If you are using Maven, add this to your pom.xml file:
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:2.4.0'
+implementation 'com.google.cloud:google-cloud-spanner-jdbc:2.4.0'
 ```
 
 If you are using SBT, add this to your dependencies