[MINOR][SQL] Fix a few function description error.

## What changes were proposed in this pull request?

Fix a few function description error.

## How was this patch tested?

manual tests

![descissues](https://cloud.githubusercontent.com/assets/5399861/26619392/d547736c-4610-11e7-85d7-aeeb09c02cc8.gif)

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18157 from wangyum/DescIssues.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/mathExpressions.scala

@@ -283,7 +283,7 @@ case class Cosh(child: Expression) extends UnaryMathExpression(math.cosh, "COSH"
       > SELECT _FUNC_('100', 2, 10);
        4
       > SELECT _FUNC_(-10, 16, -10);
-       16
+       -16
   """)
 case class Conv(numExpr: Expression, fromBaseExpr: Expression, toBaseExpr: Expression)
   extends TernaryExpression with ImplicitCastInputTypes {

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -1047,7 +1047,7 @@ case class FormatString(children: Expression*) extends Expression with ImplicitC
   """,
   extended = """
     Examples:
-      > SELECT initcap('sPark sql');
+      > SELECT _FUNC_('sPark sql');
        Spark Sql
   """)
 case class InitCap(child: Expression) extends UnaryExpression with ImplicitCastInputTypes {