Fix spelling

Doc/WhenToUse.md

@@ -25,7 +25,7 @@ Tx.Core adds the following new features to Rx:
 
 * Support for **Multiplexed** sequences (single sequence containing events of different types in order of occurence). The simplest example of turning multiplexd sequence into type-specific Obseravable-s is the [Demultiplexor](http://tx.codeplex.com/SourceControl/latest#Source/Tx.Core/Demultiplexor.cs)
 * Merging **multiple input files** in order of occurence - e.g. two log files
-* **Hereogeneous** Inputs - e.g. queries across some log files (.evtx) and some traces (.etl)
+* **Heterogeneous** Inputs - e.g. queries across some log files (.evtx) and some traces (.etl)
 * **Single-pass-read** to answer multiple queries on file(s)
 * **Scale in # of queries**. This is side effect of the above, which applies to both real-time and single-read of past history
 * Providing **virtual time** as per event timestamps. See [TimeSource](http://tx.codeplex.com/wikipage?title=TimeSource)