chore: regenerate README (#407)

This PR was generated using Autosynth. 🌈


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

```
2021-05-13 13:58:25,505 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-logging-logback/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-13 13:58:26,827 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/14b76561-1fca-4969-a1f1-4bc556569158/targets

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Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-logging-logback.git",
-        "sha": "f1e8d14b4188f6d144286204633d3979d7950baa"
+        "sha": "ca0fad04398c381430557ab03ba5601d2145ee5e"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "cfa5a9fcec63df6890279e29756602fd5858c023"
+        "sha": "bd8281a06cc7f84906e04d4843c1d3d386a980cd"
       }
     }
   ]

README.md

@@ -20,18 +20,18 @@ If you are using Maven, add this to your pom.xml file:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-logging-logback</artifactId>
-  <version>0.120.6-alpha</version>
+  <version>0.120.7-alpha</version>
 </dependency>
 ```
 
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-logging-logback:0.120.6-alpha'
+compile 'com.google.cloud:google-cloud-logging-logback:0.120.7-alpha'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.120.6-alpha"
+libraryDependencies += "com.google.cloud" % "google-cloud-logging-logback" % "0.120.7-alpha"
 ```
 
 ## Authentication