chore: regenerate README (#830)

This PR was generated using Autosynth. 🌈


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

```
2021-05-13 15:34:12,469 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-bigquerydatatransfer/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-13 15:34:13,700 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/b46808bf-b3aa-48c7-9d6c-b57a830143a9/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": "333764b1e8353790cf2461eb666a52d1020fde90"
+        "sha": "d7f114b90804970543a163593d357ddf1478a989"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "c0edb1b97d81f56eaba1e896e9756268940c56c1"
+        "sha": "bd8281a06cc7f84906e04d4843c1d3d386a980cd"
       }
     }
   ]

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>1.2.14</version>
+  <version>1.2.15</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:1.2.14'
+compile 'com.google.cloud:google-cloud-bigquerydatatransfer:1.2.15'
 ```
 
 If you are using SBT, add this to your dependencies
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigquerydatatransfer" % "1.2.14"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigquerydatatransfer" % "1.2.15"
 ```
 
 ## Authentication