Fixed: Correct the Unix command for downloading the Gradle wrapper in README (OFBIZ-13417)

The "Quick start" section of README.md documented the Unix command to
download the Gradle wrapper as an absolute path (/gradle/init-gradle-wrapper.sh),
which makes it fail with "No such file or directory" when run from the
OFBiz home directory.

Use the relative executable form (./gradle/init-gradle-wrapper.sh), consistent
with the other Unix-like commands in the README (e.g. ./gradlew,
./pullPluginSource.sh). The script is executable, so it runs directly.

Author: visvendra1991

README.md

@@ -106,7 +106,7 @@ MS Windows: `init-gradle-wrapper`
 > **Note** -
 > If you wonder where are stored the PowerShell Executables, here are the answers: [https://s.apache.org/w5dye](https://s.apache.org/w5dye)
 
-Unix-like OS: `/gradle/init-gradle-wrapper.sh`
+Unix-like OS: `./gradle/init-gradle-wrapper.sh`
 
 ### Prepare OFBiz