chore: regenerate README (googleapis#315)

This PR was generated using Autosynth. 🌈


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

```
2021-01-07 00:27:55,212 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-01-07 00:27:56,101 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/89773a7d-6719-43aa-97b1-4ac196b3ee3d/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-jdbc.git",
-        "sha": "bf2cdae2b42d3b3c3bc5bb01deee3a32e66ebd0e"
+        "sha": "5115ed8f45b4fe1c0424d25190cdfc8681ef388f"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "41e998d5afdc2c2143a23c9b044b9931936f7318"
+        "sha": "a3e990f3545dc8ccd384a75d20ce9cb185ca6a28"
       }
     }
   ]

README.md

@@ -17,17 +17,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner-jdbc</artifactId>
-  <version>1.18.2</version>
+  <version>1.18.3</version>
 </dependency>
 ```
 
-If you are using Gradle, add this to your dependencies
+If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:1.18.2'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:1.18.3'
 ```
+
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.18.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.18.3"
 ```
 
 ## Authentication