chore: regenerate README (#720)

This PR was generated using Autosynth. 🌈


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

```
2021-03-23 15:44:40,679 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigquerydatatransfer/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-03-23 15:44:41,745 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/5f43d859-9f43-4fe5-b3b0-a07b09beebad/targets

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquerydatatransfer.git",
-        "sha": "a6f619c4407c645316f2b6d40f1a76de16a21ca6"
+        "sha": "2cc1866f849b6ed0932265267eb6c935cab37483"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "79c8dd7ee768292f933012d3a69a5b4676404cda"
+        "sha": "c84c8f156e09702e1c8946bfb9746e6f5892cf27"
       }
     }
   ]

README.md

@@ -38,7 +38,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-bigquerydatatransfer</artifactId>
-  <version>1.2.2</version>
+  <version>1.2.3</version>
 </dependency>
 
 ```
@@ -51,12 +51,12 @@ compile 'com.google.cloud:google-cloud-bigquerydatatransfer'
 ```
 If you are using Gradle without BOM, add this to your dependencies
 ```Groovy
-compile 'com.google.cloud:google-cloud-bigquerydatatransfer:1.2.2'
+compile 'com.google.cloud:google-cloud-bigquerydatatransfer:1.2.3'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigquerydatatransfer" % "1.2.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigquerydatatransfer" % "1.2.3"
 ```
 
 ## Authentication