[SQL] Update SQLContext.read.text doc

Since we rename the column name from ```text``` to ```value``` for DataFrame load by ```SQLContext.read.text```, we need to update doc.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10349 from yanboliang/text-value.

python/pyspark/sql/readwriter.py

@@ -207,7 +207,7 @@ def parquet(self, *paths):
     @ignore_unicode_prefix
     @since(1.6)
     def text(self, paths):
-        """Loads a text file and returns a [[DataFrame]] with a single string column named "text".
+        """Loads a text file and returns a [[DataFrame]] with a single string column named "value".
 
         Each line in the text file is a new row in the resulting DataFrame.
 

sql/core/src/main/scala/org/apache/spark/sql/DataFrameReader.scala

@@ -339,7 +339,7 @@ class DataFrameReader private[sql](sqlContext: SQLContext) extends Logging {
   }
 
   /**
-   * Loads a text file and returns a [[DataFrame]] with a single string column named "text".
+   * Loads a text file and returns a [[DataFrame]] with a single string column named "value".
    * Each line in the text file is a new row in the resulting DataFrame. For example:
    * {{{
    *   // Scala:

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/text/DefaultSource.scala

@@ -76,7 +76,7 @@ private[sql] class TextRelation(
     (@transient val sqlContext: SQLContext)
   extends HadoopFsRelation(maybePartitionSpec, parameters) {
 
-  /** Data schema is always a single column, named "text". */
+  /** Data schema is always a single column, named "value". */
   override def dataSchema: StructType = new StructType().add("value", StringType)
 
   /** This is an internal data source that outputs internal row format. */