Fix typo in plan docs

Author: danielabrozzoni

src/plan.rs

@@ -10,7 +10,7 @@
 //! doing coin selection. Furthermore it provides which subset of those keys and hash pre-images you
 //! will actually need as well as what locktime or sequence number you need to set.
 //!
-//! Once you've obstained signatures, hash pre-images etc required by the plan, it can create a
+//! Once you've obtained signatures, hash pre-images etc required by the plan, it can create a
 //! witness/script_sig for the input.
 
 use core::cmp::Ordering;