chore: regenerate README (#296)

This PR was generated using Autosynth. 🌈


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

```
2020-12-11 06:52:44,370 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner-jdbc/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2020-12-11 06:52:45,353 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/7cbd341c-d26d-4b29-b8c7-fa75aeee0c64/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": "9eea34c4206845a5e23b5b6ac78509497fd06069"
+        "sha": "53b459e334fdbf391d080817f8b782cff3b12d1b"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "352f25621c28d2fa0784984ea0ed5ea697aa1525"
+        "sha": "996775eca5fd934edac3c2ae34b80ff0395b1717"
       }
     }
   ]

README.md

@@ -17,17 +17,17 @@ 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>1.17.3</version>
+  <version>1.18.0</version>
 </dependency>
 ```
 
 If you are using Gradle, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-spanner-jdbc:1.17.3'
+compile 'com.google.cloud:google-cloud-spanner-jdbc:1.18.0'
 ```
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.17.3"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner-jdbc" % "1.18.0"
 ```
 
 ## Authentication