Update doc/OnlineDocs/explanation/modeling_utils/external_functions/index.rst

Co-authored-by: Bethany Nicholson <blnicho@users.noreply.github.com>

Author: eslickj

doc/OnlineDocs/explanation/modeling_utils/external_functions/index.rst

@@ -98,7 +98,7 @@ This function is a signed square root approximation defined as:
         \sum_{i=0}^{11} c_i x^i & \text{if } |x| < 0.1
     \end{cases}
 
-This function is $C^4$ smooth.  The region :math:`-0.1 < x < 0.1` is replaced by an 11th order polynomial approximates that :math:`\text{sgn}(x)|x|^{0.5}`.  The approximate function has a well behaved derivatives at :math:`x=0`.  If you need to use this function with very small numbers and high accuracy is important, you can scale the argument up (e.g. :math:`\text{sgnsqrt\_c4}(sx)/s^{0.5}`).
+This function is :math:`C^4` smooth.  The region :math:`-0.1 < x < 0.1` is replaced by an 11th order polynomial that approximates :math:`\text{sgn}(x)|x|^{0.5}`.  This function has well behaved derivatives at :math:`x=0`.  If you need to use this function with very small numbers and high accuracy is important, you can scale the argument up (e.g. :math:`\text{sgnsqrt_c4}(sx)/s^{0.5}`).
 
 These figures show the signed square root function compared to the smooth approximation sgnsqr_c4(x).