Add links to tickets in fork doc (apache-spark-on-k8s#421)

Author: sjrand

FORK.md

@@ -1,19 +1,19 @@
 # Difference with upstream
 
-* SPARK-21195 - Automatically register new metrics from sources and wire default registry 
-* SPARK-20952 - ParquetFileFormat should forward TaskContext to its forkjoinpool
-* SPARK-20001 (SPARK-13587) - Support PythonRunner executing inside a Conda env (and R)
-* SPARK-17059 - Allow FileFormat to specify partition pruning strategy via splits
-* SPARK-24345 - Improve ParseError stop location when offending symbol is a token
-* SPARK-23795 - Make AbstractLauncher#self() protected 
-* SPARK-23153 - Support application dependencies in submission client's local file system
-* SPARK-18079 - CollectLimitExec.executeToIterator should perform per-partition limits
+* [SPARK-21195](https://issues.apache.org/jira/browse/SPARK-21195) - Automatically register new metrics from sources and wire default registry
+* [SPARK-20952](https://issues.apache.org/jira/browse/SPARK-20952) - ParquetFileFormat should forward TaskContext to its forkjoinpool
+* [SPARK-20001](https://issues.apache.org/jira/browse/SPARK-20001) ([SPARK-13587](https://issues.apache.org/jira/browse/SPARK-13587)) - Support PythonRunner executing inside a Conda env (and R)
+* [SPARK-17059](https://issues.apache.org/jira/browse/SPARK-17059) - Allow FileFormat to specify partition pruning strategy via splits
+* [SPARK-24345](https://issues.apache.org/jira/browse/SPARK-24345) - Improve ParseError stop location when offending symbol is a token
+* [SPARK-23795](https://issues.apache.org/jira/browse/SPARK-23795) - Make AbstractLauncher#self() protected
+* [SPARK-23153](https://issues.apache.org/jira/browse/SPARK-23153) - Support application dependencies in submission client's local file system
+* [SPARK-18079](https://issues.apache.org/jira/browse/SPARK-18079) - CollectLimitExec.executeToIterator should perform per-partition limits
 
-* SPARK-15777 (Partial fix) - Catalog federation
+* [SPARK-15777](https://issues.apache.org/jira/browse/SPARK-15777) (Partial fix) - Catalog federation
  * make ExternalCatalog configurable beyond in memory and hive
  * FileIndex for catalog tables is provided by external catalog instead of using default impl
 
-* Better pushdown for IN expressions in parquet via UserDefinedPredicate (SPARK-17091 for original issue)
+* Better pushdown for IN expressions in parquet via UserDefinedPredicate ([SPARK-17091](https://issues.apache.org/jira/browse/SPARK-17091) for original issue)
 
 # Added