Fix typo

appf.adoc

@@ -88,7 +88,7 @@ __Notes:__:: The geostationary projection assumes a hypothetical view of the Ear
 This model is independent of the physical scan principles of any observing instrument.
 The model consists conceptually of a set of two rotating circles with a colocated centre, whose axes of rotation are perpendicular to each other.
 The axis of the outer circle is stationary, while the axis of the inner circle moves about the stationary axis.
-This means that a given viewing angle described using thsi model is the result of matrix multiplications, which is not commutative, so that order of operations is essential in achieving accurate results.
+This means that a given viewing angle described using this model is the result of matrix multiplications, which is not commutative, so that order of operations is essential in achieving accurate results.
 
 +
 The algorithm for computing the mapping may be found at link:$$http://www.cgms-info.org/documents/pdf_cgms_03.pdf$$[http://www.cgms-info.org/documents/pdf_cgms_03.pdf].