gh-36062: Fix typo in docstring
    
This corrects a typo in a docstring concerning subgroups of the modular
group.  The typo made a definition mathematically incorrect.  See issue
#36058


- [x ] The title is concise, informative, and self-explanatory.
- [ x] The description explains in detail what this PR is about.
- [ x] I have linked a relevant issue or discussion.

- [ x] I have updated the documentation accordingly.

### ⌛ Dependencies

No dependencies: based on 10.1.beta9
    
URL: #36062
Reported by: John Cremona
Reviewer(s): David Coudert

Author: Release Manager

src/sage/modular/arithgroup/congroup_gammaH.py

@@ -40,7 +40,7 @@ def GammaH_constructor(level, H):
     r"""
     Return the congruence subgroup `\Gamma_H(N)`, which is the subgroup of
     `SL_2(\ZZ)` consisting of matrices of the form `\begin{pmatrix} a & b \\
-    c & d \end{pmatrix}` with `N | c` and `a, b \in H`, for `H` a specified
+    c & d \end{pmatrix}` with `N | c` and `a, d \in H`, for `H` a specified
     subgroup of `(\ZZ/N\ZZ)^\times`.
 
     INPUT:
@@ -163,7 +163,7 @@ class GammaH_class(CongruenceSubgroup):
     The congruence subgroup `\Gamma_H(N)` for some subgroup `H \trianglelefteq
     (\ZZ / N\ZZ)^\times`, which is the subgroup of `\SL_2(\ZZ)` consisting of
     matrices of the form `\begin{pmatrix} a &
-    b \\ c & d \end{pmatrix}` with `N \mid c` and `a, b \in H`.
+    b \\ c & d \end{pmatrix}` with `N \mid c` and `a, d \in H`.
 
     TESTS: