Tweak timings of exercises to better reflect teaching times (#1839)

A few of the early exercises had much larger estimates than were
actually necessary for the super simple early exercises. I've gone
through and reviewed the time estimates for exercises and tweaked the
estimates based on how much time students have actually needed in my
classes so far.

src/borrowing/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 30
+minutes: 20
 ---
 
 # Exercise: Health Statistics

src/control-flow-basics/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 30
+minutes: 15
 ---
 
 # Exercise: Collatz Sequence

src/error-handling/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 20
+minutes: 30
 ---
 
 # Exercise: Rewriting with Result

src/references/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 30
+minutes: 15
 ---
 
 # Exercise: Geometry

src/std-types/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 10
+minutes: 20
 ---
 
 # Exercise: Counter

src/tuples-and-arrays/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 30
+minutes: 15
 ---
 
 # Exercise: Nested Arrays

src/types-and-values/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 30
+minutes: 15
 ---
 
 # Exercise: Fibonacci